The present invention relates to a burner with toroidal-cyclone flow for a liquid and gas fueled boiler having a tubular body, composed of an outer shell and an inner shell mounted coaxially with the outer shell, a fuel supply nozzle located coaxially inside these shells, and ignition electrodes, wherein the inner shell and the outer shell define an annular gap between them provided with a constricted circular opening in front of the burner.
Various types of burners are already known in which the comburant and fuel gases are mixed downstream of the nozzle such as to generate two-phase combustion intended to improve the quality of combustion obtained in this burner. Burners of this type often allow combustion to be significantly improved, but without achieving the expected results, in particular low enough nitrogen oxide and carbon dioxide levels for the combustion gases to be below the tolerances set by current and future codes.
It should be pointed out that clean, complete combustion of a liquid or gaseous fuel mixed with a comburant gas, namely air, can be achieved only if the following three conditions are fulfilled:
a) The fuel must be divided into extremely fine particles;
b) The fuel-comburant mixture must be in very definite proportions;
c) Guidance of the fluids must be ensured to allow complete mixing of the substances involved and generate a fluid-dynamics flow of combustion gas.
In a burner of the "Low NOx" type, it is essential to achieve combustion in two phases. The first phase consists of starting combustion with a rich mixture and the second phase consists of carrying out this combustion under conditions approaching stoichiometric conditions. With regard to the problem of particulate division of the fuel, it is known that optimum combustion is only possible if the fuel is in the form of extremely fine particles. It is also known that a stratified mixture of fuel and comburant must be obtained, and for this it is essential to maximize use of the effects resulting from the flow of these fluids and turbulence with a small pressure loss inside the burner. In general, too much air cools the flame and handicaps combustion. Too little air, on the other hand, leaves unburnt gases and favors formation of carbon monoxides. If fuel-comburant mixing is done poorly, i.e. if the resulting mixture is not stratified, it is not possible to achieve a mixing coefficient minimizing the quantities of harmful substances and/or pollutants in the combustion gases. In this case, the flame obeys its own laws of dynamic behavior. When the mixture is stratified, however, and is composed of air and finely divided fuel particles burned in the first combustion phase, the flame burns at a fully predetermined rate. To obtain a stable flame, the mixture must be created with precision and fed at a constant rate corresponding to the speed of the flame front. For combustion to be complete and the efficiency of the flame to be at a maximum with no undesirable residues appearing in the combustion gases, it is necessary for the adjustments defining turbulences and residence times of the gases in the burner to be fully defined. Recycling of combustion gases has beneficial advantages since it allows the mixture to be preheated, the residence time of the gases in the burner to be increased, and favorable influences to be exerted on the chemical reactions that occur. This recycling must however be done very carefully and the quantity of recycled gases must be optimally specified as a function of the quantities of primary, secondary, and tertiary air injected.
Burners known to date do not in general allow the various parameters to be optimally reconciled so that the results of combustion gas analyses generally give values over the threshold permitted by current and future regulations, or flame oscillations often occur with fouling of the device, or they are of exorbitant cost.
The present invention proposes to mitigate these disadvantages by providing a burner of relatively simple and economical construction which causes a toroidal-cyclone flow to be created with direct recycling of the combustion gases, which has the effect of achieving an extremely fine division of the fuel particles, a stratified mixture of the fuel and comburant gas, in this case air, efficient recycling of the combustion gases, and hence a sufficiently long residence time of the gases in the burner for unburnt residues to be virtually nonexistent.